Apparatus and method for producing amorphous silica ash

ABSTRACT

The invention provides apparatus and a method for exothermic treatment of siliceous feed material to provide an ash having a predominantly silica content, typically more than 97% by weight. The apparatus includes a cylindrical housing extending about a central axis and having first and second ends, and a central mixing zone adjacent the first end. A material intake carries the feed material into the housing and has an outlet in the mixing zone, and gas enters through a first gas inlet at the first end through guides to create an inner vortex extending axially about said axis. Gas also enters through guides in a second gas inlet at the second end to create an outer vortex which also extends axially but in the opposite direction from that of the inner vortex. However the inner and outer vortices rotate in the same direction. A gas outlet is positioned to receive spent gas from the housing and an ash outlet is positioned remotely from the mixing zone to receive the ash. If preferred, the position of the ash outlet may be incorporated into the gas outlet. A control system is provided to limit the temperature in the feed material in the housing. As a result, in operation, gases can be fed into the first and second gas inlets carrying sufficient oxygen for exothermic combustion of the feed material and having a flow rate to create said inner and outer vortices. The outer vortex meets the inner vortex in the mixing zone to mix both with the inner vortex and with the feed material which will be subjected to exothermic combustion as the feed material is entrained in the inner and outer vortices. The material will pass through the mixing zone repeatedly until the feed material is converted to an ash having a predominantly amorphous silica content and escape criteria needed to reach and exit through the ash outlet.

FIELD OF THE INVENTION

[0001] This invention relates to removing carbon and volatiles fromsiliceous materials to produce a predominantly amorphous silica ashhaving a relatively low carbon content.

BACKGROUND OF THE INVENTION

[0002] Agricultural products create waste materials such as rice hulls,rice straw, wheat chaff, and straw that are relatively high in siliceouscontent. It is well known that these waste materials would be usefulindustrially if the carbon content could be removed efficiently andeconomically to produce an ash having a predominantly amorphous silicacontent with less than 3% carbon. Clearly a process which would producesuch material would have the added benefit of redirecting wastematerials from disposal facilities to a useful industry.

[0003] In general, the materials vary in their physical characteristicssuch as shape, density, water content, and inclusion of tramp andextraneous mineral matter, and they also exhibit relatively lowcalorific values when compared with liquid or gaseous hydrocarbons.

[0004] Rice hulls are an example of such waste materials and theydemonstrate the type of problems associated with the disposal of similarwaste materials. Although rice hulls have found some minor uses such asfuel in low calorific production of energy, and as a cover to excludeoxygen from the surface of molten steel, large amounts of rice hullwaste can be found around the world making disposal a particularproblem.

[0005] Clearly there is a need to find economically feasible uses forthese waste materials that are inevitable by-products of the productionof products that will continue to be in high demand.

[0006] It is known that amorphous silica ash would be desirable as apozzolan in concrete if the ash could be made to have a silica contentin excess of 97% by weight with minor amounts of crystalline silica inthe order of less than 1% of the total silica. The remaining 3% will bemade up of carbon and some trace elements To achieve this resultefficiently in a commercial process, the waste material would have to beincinerated at an elevated temperature sufficient to burn off the carbonand volatiles efficiently and yet avoid hot spots which will result inincomplete combustion due to encapsulation of carbon by the formation ofglassy coatings. If the temperature is allowed to pass a critical levelin any part of the process, the silica will agglomerate as it becomestacky with detrimental results for the apparatus, and the internal poresurface area of the ash paticles will be reduced. Also, if thetemperature is further elevated locally, silica in that area willconvert to a crystalline structure which is dangerous to handle, and theresulting product will be unsuitable for use as a pozzolan.

[0007] Accordingly, it is among the objects of this invention to provideapparatus and methods for treating siliceous waste materials efficientlyto produce amorphous silica ash having low carbon content and minimalcrystalline structure so that the ash can be used as a pozzolan,particularly in concrete.

SUMMARY OF THE INVENTION

[0008] In one of its aspects, the invention provides apparatus forexothermic treatment of siliceous feed material to provide an amorphoussilica ash. The apparatus includes a cylindrical housing extending abouta central axis and having first and second ends, and a central mixingzone adjacent the first end. A material intake carries the feed materialinto the housing and has an outlet in the mixing zone, and gas entersthrough a first gas inlet at the first end through guides to create aninner vortex extending axially about said axis. Gas also enters throughguides in a second gas inlet at the second end to create an outer vortexwhich alkso extends axially but in the opposite direction from that ofthe inner vortex. However the inner and outer vortices rotate in thesame direction. A gas outlet is positioned to receive spent gas from thehousing and an ash outlet is positioned remotely from the mixing zone toreceive the ash. If preferred, the position of the ash outlet may beincorporated into the gas outlet. A control system is provided to limitthe temperature in the feed material in the housing. As a result, inoperation, gases can be fed into the first and second gas inletscarrying sufficient oxygen for exothermic combustion of the feedmaterial and having a flow rate to create said inner and outer vortices.The outer vortex meets the inner vortex in the mixing zone to mix bothwith the inner vortex and with the feed material which will be subjectedto exothermic combustion as the feed material is entrained in the innerand outer vortices. The material will pass through the mixing zonerepeatedly until the feed material is converted to a predominantlyamorphous silica ash having escape criteria needed to reach and exitthrough the ash outlet.

[0009] In another of its aspects, the invention provides a method ofmaking amorphous silica ash by feeding exothermic siliceous materialinto a mixing zone for eventual discharge through an ash outlet, andproviding gas flow as inner and outer vortices moving about a commonaxis and in opposite axial directions with the same angular motion. Thegas flow contains sufficient oxygen for exothermic combustion of thefeed material. The vortices meet in the mixing zone to carry the feedmaterial axially in the inner vortex with a centrifugal force componentto cause the material to move outwardly from the inner vortex into theouter vortex so that the material is entrapped in the vortices andpassed repeatedly through the mixing zone until the feed material isconverted to amorphous silica ash having escape criteria needed to reachthe ash outlet so that the ash will then exit through the ash outlet.The temperatures in the gas flow are monitored by a control system tooperate the method so that the feed material is subjected to no morethan maximum temperatures selected to result in predominantly amorphoussilica ash.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be better understood with reference to theaccompanying drawings, in which:

[0011]FIG. 1 is a schematic cross-sectional view of apparatus accordingto a preferred embodiment of the invention and showing the apparatus ina vertical orientation for use in accordance with a preferred embodimentof a method according to the invention: and

[0012]FIG. 2 is a diagrammatic side view of part of a guide used in theapparatus to create vortex gas flow.

DETAILED DESCRIPTION OF THE INVENTION

[0013] Reference is first made to FIG. 1 to describe apparatus accordingto the invention and designated generally by the numeral 20. Theapparatus will be described with reference to the treatment of feedmaterial 22 that enters through a material intake 24 and falls down afeed tube 26 to exit annularly at a flared bottom outlet 28. The outlet28 is located in an annular mixing zone designated generally by thenumeral 30 and provided to mix the incoming feed material both withflowing gas and with feed material which is already entrapped in the gasflow, as will be described.

[0014] The apparatus 20 includes a cylindrical housing 32 that extendsvertically about a central axis 33 that is also the axis of the feedtube 26. A bottom 34 of the housing 32 is frusto-conical, taperingdownwardly to meet a first gas inlet 35. This inlet 35 consists of aninlet pipe 36 leading the incoming gas upwardly to an annular path 38about a central divider 40 to bring the gas into engagement with anannular guide 42 at the bottom 34 of the housing 32. The annular guide42 consists of a series of spaced blades 44 arranged as shown in FIG. 2to cause the gas to leave the guide 42 in a helical path or vortex assuggested by arrows 45. This vortex is an inner vortex and sets out tomove vertically through the mixing zone 30 and along the axis 33 of thehousing 32 as indicated by the arrows 46.

[0015] The mixing zone 30 is also affected by an outer vortex indicatedby arrows 47 and created by gas supplied from a second gas inlet 48having an inlet pipe 50 leading to a top manifold 52 feeding an annularguide 54 similar to guide 42 at the bottom of the housing 32. Theannular guide 54 is positioned to develop an outer vortex having thesame direction of angular rotation as the inner vortex. This outervortex moves downwardly following the inner surface of a cylindricalouter wall 55 of the housing 32 before being deflected inwardly towardsthe annular mixing zone by the frusto-conical bottom 34. As a result theouter vortex meets both the inner vortex and the feed material 22 in themixing zone 30 before the inner vortex, reinforced by the gas from theouter vortex, starts upwardly towards a spent gas outlet 59 whilecarrying feed material 22 with it in the inner vortex. This reinforcedinner vortex is indicated by arrows 56.

[0016] In order to better understand the process it is convenient tostart by considering a new supply of feed material 22 entering themixing zone 30 from the bottom outlet 28 of the feed tube 26. Thematerial will be caught up in the gas flow at the reinforced innervortex where it will pick up a velocity which has both an upwardvertical component and an outward horizontal component caused by thecentrifugal effect of the vortex. As a piece of this new feed materialrises in the gas flow, the horizontal component will cause the piece toescape the inner vortex and move outwardly into the downwardly movingouter vortex as indicated by the arrows 57. Once the material reachesthe outer vortex, it will move downwardly with the vortex to return tothe mixing zone where it will impact with new pieces of feed materialand again enter the inner vortex.

[0017] However there are other factors at work in the mixing zone 30because the process is exothermic and the feed material 22 will combustas it travels. This combustion is initiated in any convenient mannersuch as by the use of a starter flame carried by a lance indicated inghost outline at 58. Once the combustion starts, it will continue aslong as new feed material 22 is fed into the inner vortex through theoutlet 28 and at a rate sufficient to maintain combustion. Of course thegases supplied through the first and second gas inlets must togethercarry sufficient oxygen to support combustion. As will be explained, therate of introduction of feed material 22 is controlled to maintainselected temperatures in the combustion to ensure that the result is anash of the required quality.

[0018] As a result of combustion, the feed material reentering themixing zone from the outer vortex will be hot and the new materialcoming from the feed tube will be at a lower temperature. This willfurther cause stresses in the material to assist the mechanical impactsin breaking up the material. Some of the mixture of old and new materialwill travel with the inner vortex above the arrows 57 beforetransferring to the inner vortex at a level such as that indicated byarrows 60. The material will then to return to the mixing zone 30 andthis will be repeated so long as the material escapes from the innervortex at a level below an ash outlet 62 provided in the wall 55 tocollect ash which has the required characteristics needed to reach theheight of the ash outlet 62. Consequently, any material that has notcombusted to lose the required amount of carbon will tend to fall backinto the outer vortex along paths such as those indicated by arrows 57and 60 whereas amorphous silica ash which has developed the necessaryescape criteria will pass along a path such as that shown by arrows 64,(i.e. above the ash outlets 62) before finding its way through one ofthe ash outlets 62.

[0019] It will be appreciated that some small fines of ash will becomeentrapped in the spent gas leaving through the spent gas outlet 59. Infact, the apparatus can be operated by using this as the outlet for theash and then later trapping the ash in a separator indicated by thenumeral 66. Collected ash is then taken from the separator 66 through abottom port 68 in the separator 66.

[0020] It is important to note that as feed material enters the mixingzone 30, the stresses on the material will cause separation betweenparticles and this will enhance the combustion. Also, because thecombustion is more uniform, the combustion temperature is also moreuniform in the inner vortex. This tends to limit hot spots which couldheat the silica to a level where carbon would be trapped in theparticle. As a result the carbon and volatile compounds are combustedleaving only the silica provided that the temperature in the inner andouter vortices is controlled below an optimum temperature. If thetemperature is slightly too high, the silica will become tacky and theparticles may agglomerate and tend to remain in the apparatus.Similarly, if the temperature is higher again, undesirable crystallinesilica will be formed in unacceptable quantities.

[0021] The apparatus also includes a control system 69 to maintain atemperature at which the ash will form without contamination by carbonor crystalline silica while at the same time allowing the temperature toreach a level where the residence time of the feed material in theapparatus is minimised.

[0022] The control system 69 includes one or more thermocouples 70connected to a controller 72 which is supplied with power by lines 74.The controller 72 compares known parameters for the apparatus withreadings taken from the thermocouples 70 to determine whether or not therate of flow of feed material 22 should be changed. If the temperatureis too low, more material will be needed to support combustion at thedesired temperature, whereas if it is too high, the rate of flow of feedmaterial should be lowered to allow less combustion and hence a lowertemperature. To achieve this, signals are sent from the controller 72 bylines 76 to a flow rate controller 78 in the material intake 24 to varythe rate of flow of feed material 22 from a hopper 80 to the feed tube26.

[0023] Clearly, since the temperature of combustion is controlled byvarying the rate of flow of feed material 22, there must be excessoxygen provided at all times in the flow of gas through the first andsecond gas inlets 35 and 48.

[0024] The preferred embodiments described with reference to thedrawings can be varied within the scope of the invention. For instance,because the vortices are not affected greatly by gravity, the apparatuscan be arranged in any orientation including horizontally. Also, theguides shown in FIG. 2 can be replaced by any suitable structure thatwill create the inner and outer vortices. For instance a series ofannular nozzles arranged to point tangentially and axially would give asimilar result. Accordingly, the term ‘guide’ as used in thisdescription, and in the claims, is intended to include such mechanicalequivalents. These and other variations are within the scope of theinvention as described and claimed.

[0025] It has been found that various parameters must be regulated bythe control system for a given waste material and for the apparatus. Forexample, in order for a low carbon, high amorphous silica ash to beproduced from rice hull waste, it has been found that the processtemperature used in combustion in the inner vortex (as measured by athermocouple inserted within the combusting mass of hull particles) isideally in the range 830-850° C., preferably in the range 750-875° C.The higher the temperature, the faster the combustion process iscompleted. However, as the temperature goes above 850° C., accretion ofthe silica in the ash causes agglomeration followed by a transition tocrystalline silica. Thus very close control of the combustion process iscritical for efficient operation.

[0026] As the rice hull is combusted, it loses some 80% of its originalweight, so that there is a tendency for the rice hull to be prematurelyentrained out of the combustion system due to the reduced terminalvelocity of the ash particle. This tendency is controlled by theselection of the velocities of the inner and outer vortices to allowonly those particles of ash that meet designed escape criteria to becomeentrained for escape. This ensures that the apparatus retains the ashuntil the required percentage of carbon has been removed leavingamorphous silica having a low carbon content.

[0027] There is an initial heating process which takes place as the ricehulls are heated to produce a carbon rich ash which is then subjected tocombustion in the desired temperature range while maintaining the ash inthe apparatus despite the weight loss.

[0028] The use of inner and outer vortices in the manner describedprovides a well mixed flow of feed particles which remain in circulationbetween the vortices until sufficiently processed. The combination ofthe inner and outer vortices spinning in the same horizontal directioncreates a turbulent and controlled mixing pattern resulting in improvedparticle separation characteristics. The particle size exhausted fromthe apparatus is <40 micrometers and the silica content is in excess of97% with the remainder made up primarily of residual carbon withresidual materials which are of no significance in the finished ash.

[0029] It was found that apparatus according to the invention is capableof processing whole rice hulls of varying water content, controlling thecombustion reaction temperature ±10° C. and retaining the ash until thecarbon had been sufficiently combusted. By this means a commerciallyacceptable ash can be produced in quantities that meet all theacceptance criteria for pozzolans to be used in concrete.

[0030] The accumulation of tramp was not a problem because with carefulselection of the inlet velocity of the inner vortex, the larger tramp orincombustible particles accumulated at the bottom of the apparatus. Ifneeded an outlet could be provided for these materials at the bottom forperiodic removal.

[0031] It is also possible to add a small percentage of hard inertparticles to the feed material (such as silicon carbide, alumina,zirconia) to remain in the apparatus to carry out an air milling actionthereby further reducing the produced particle size.

[0032] In the event that the feed material is not suitable for directentry into the apparatus it may be necessary to chop or shred thematerial into smaller sizes before entry. However it has been found thatthe apparatus will accommodate small percentages of chopped strawthereby illustrating the capability of the device to process chopped,stringy or shredded materials.

[0033] It will also be evident that the results in using the apparatusand practicing the method of the invention will depend on how theapparatus is managed. The apparatus and the method have been usedsuccessfully to produce ash from waste materials (such as rice hull ash)to give an ash which has less than 3% by weight of carbon, and less than1% of the silica is in the crystalline form. In fact, carbon content hasbeen less than 1% and the crystalline carbon has been limited to traceamounts. These results were achieved with no more than due care andattention to the proper operation of the apparatus and the method.

[0034] It will now be apparent that the apparatus and method of theinvention can be varied within the teaching of the invention, and thatsuch variations are within the scope of the invention as claimed.

1. Apparatus for exothermic treatment of siliceous feed material, theapparatus including: a cylindrical housing extending about a centralaxis and having first and second ends, and a central mixing zoneadjacent the first end; a material intake for carrying said feedmaterial into the housing and having a bottom outlet in the mixing zone;a flow rate controller coupled to the material intake to vary the rateof flow of feed material through the intake; a first gas inlet at saidfirst end and having guides to create an inner vortex extending aboutsaid axis; a second gas inlet at said second end and having guides tocreate an outer vortex extending about said axis, the inner and outervortices rotating in the same direction about said axis and movingaxially in opposite directions to create a gas stream; a gas outletpositioned to receive spent gas from the housing; an ash outletpositioned remotely from the mixing zone; a control system havingtemperature sensors in the gas stream and operable in response tochanges in temperatures in the gas stream to operate the flow ratecontroller to vary the rate of flow of the feed material into the mixingzone in accordance with temperature parameters set to limit thetemperature in the feed material in the gas stream; whereby in operationgases are fed into the first and second gas inlets to create the gasstream with sufficient oxygen for exothermic combustion of the feedmaterial and having a flow rate to create said inner and outer vortices,the outer vortex meeting the inner vortex in the mixing zone to mix bothwith the inner vortex and with the feed material leaving the materialintake, so that the feed material will be subjected to exothermiccombustion as the feed material is entrained in the gas stream to passthrough the mixing zone repeatedly until the feed material is convertedpredominantly to amorphous silica ash having escape criteria needed toexit through the ash outlet.
 2. Apparatus as claimed in claim 1 in whichthe ash outlet and the gas outlet are positioned such that said ash isentrained in the spent gas.
 3. Apparatus as claimed in claim 1 in whichthe material intake extends axially about said axis inside the innervortex.
 4. Apparatus as claimed in claim 1 in which said guides in thefirst gas inlet are blades set in an annular array about said axis. 5.Apparatus as claimed in claim 1 in which said guides in the second gasinlet are blades set in an annular array about said axis.
 6. Apparatusas claimed in claim 4 in which said guides in the second gas inlet areblades set in an annular array about said axis.
 7. Apparatus as claimedin claim 1 in which the ash outlet is adjacent said first end such thatash having said escape criteria will find the ash outlet as such ashmoves radially between the inner vortex and the outer vortex. 8.Apparatus for exothermic treatment of siliceous feed material, theapparatus including: a cylindrical housing extending about a centralaxis and having first and second ends, and a central mixing zoneadjacent the first end; a material intake for carrying said feedmaterial into the housing and having a bottom outlet in the mixing zone;a flow rate controller coupled to the material intake to vary the rateof flow of feed material through the intake; a first gas inlet at saidfirst end and having guides to create an inner vortex extending aboutsaid axis; a second gas inlet at said second end and having guides tocreate an outer vortex extending about said axis, the inner and outervortices rotating in the same direction about said axis and movingaxially in opposite directions to create a gas stream; a gas outletpositioned to receive spent gas from the housing; a control systemhaving temperature sensors in the gas stream and operable in response tochanges in temperatures in the gas stream to operate the flow ratecontroller to vary the rate of flow of the feed material into the mixingzone in accordance with temperature parameters set to limit thetemperature in the feed material in the gas stream; whereby in operationgases are fed into the first and second gas inlets to create a gasstream and carrying sufficient oxygen for exothermic combustion of thefeed material and having a flow rate to create said inner and outervortices, the outer vortex meeting the inner vortex in the mixing zoneto mix both with the inner vortex and with the feed material leaving thematerial intake, so that the feed material will be subjected toexothermic combustion as the feed material is entrained in the gas sreamto pass through the mixing zone repeatedly until the feed material isconverted predominantly to amorphous silica ash having escape criterianeeded to exit through the gas outlet with the spent gas.
 9. Apparatusas claimed in claim 8 and further including a separator coupled to thegas outlet to separate the ash from the spent gas.
 10. A method ofmaking an ash having a predominantly amorphous silica content, themethod including the steps: feeding exothermic siliceous material into amixing zone for eventual discharge through an ash outlet; creating innerand outer vortices about an axis, the vortices defining a gas streamcontaining sufficient oxygen for exothermic combustion of the feedmaterial and being arranged to flow in opposite axial directions and inthe same angular direction, the vortices meeting in the mixing zone tocarry the feed material axially in the inner vortex with a centrifugalforce component to cause the material to move outwardly from the innervortex into the outer vortex so that the material is entrapped in thegas stream and passed repeatedly through the mixing zone until the feedmaterial is converted predominantly to amorphous silica ash havingescape criteria needed to travel axially to the ash exit so that suchash will exit through the ash outlet; monitoring temperatures in the gasstream; comparing the temperatures in the gas stream with knowninformation to provide an output signal; and using the output signal tocontrol the rate of flow of the exothermic feed material into theapparatus so that the feed material in the gas stream is subjected tomaximum temperatures selected to result in an ash having a predominantlyamorphous silica content.
 11. A method as claimed in claim 10 in whichthe ash has a silica content of no less than 97% by weight. 12.Apparatus for exothermic treatment of siliceous feed material, theapparatus including: a cylindrical housing extending axially about acentral axis and having first and second ends, and a central mixing zoneadjacent the first end; a material intake for carrying said feedmaterial into the housing and having an outlet in the mixing zone; afirst gas inlet at said first end and having guides to create an innervortex extending axially; a second gas inlet at said second end andhaving guides to create an outer vortex extending about said axis, theinner and outer vortices rotating in the same direction about said axisand moving axially in opposite directions to define a gas stream; a gasoutlet positioned centrally in said second end to receive spent gas fromthe housing; an ash outlet positioned remotely from the mixing zone; acontrol system having temperature sensors in the gas flow and operablein response to changes in temperatures in the gas flow to limit thetemperature in the feed material in the housing; whereby in operationgases are fed into the first and second gas inlets carrying sufficientoxygen for exothermic combustion of the feed material and having a flowrate to create said inner and outer vortices, the outer vortex meetingthe inner vortex in the mixing zone to mix both with the inner vortexand with the feed material leaving the material intake, so that the feedmaterial will be subjected to exothermic combustion as the feed materialis entrained in the gas stream to pass through the mixing zonerepeatedly until the feed material is converted predominantly toamorphous silica ash having escape criteria needed to leave the gasstream and exit through the ash outlet.
 13. A method of making amorphoussilica ash, the method including the steps: feeding exothermic siliceousmaterial into a mixing zone for eventual discharge through an ashoutlet; creating a gas stream consisting of inner and outer vorticesabout a common axis and containing sufficient oxygen for exothermiccombustion of the feed material, the vortices meeting in the mixing zoneto carry the feed material axially in the inner vortex with acentrifugal force component to cause the material to move outwardly fromthe inner vortex into the outer vortex so that the material is entrappedin the gas stream and passed repeatedly through the mixing zone untilthe feed material is converted primarily to amorphous silica ash havingescape criteria needed to remain in the inner vortex to carry the ash toash outlet for passage through the ash outlet; collecting spent gas at aspent gas outlet; monitoring the temperatures in the gas stream;comparing the temperatures in the gas stream with known information toprovide an output signal; and using the output signal to control themaximum temperatures in the gas stream to result in a predominantlyamorphous silica ash.
 14. Apparatus as claimed in claim 12 in which theash outlet and the gas outlet are positioned such that said amorphoussilica ash is entrained in the spent gas.
 15. Apparatus as claimed inclaim 12 in which the material intake extends about said axis. 16.Apparatus as claimed in claim 12 in which said guides in the first gasinlet are blades set in an annular array about said axis.
 17. Apparatusas claimed in claim 12 in which said guides in the second gas inlet areblades set in an annular array about said axis.
 18. Apparatus as claimedin claim 16 in which said guides in the second gas inlet are blades setin an annular array about said axis.
 19. Apparatus as claimed in claim12 in which the ash outlet is adjacent and below said top such that ashwill find the ash outlet as the ash moves radially outwards to leave theinner vortex and enter the outer vortex.
 20. Apparatus for exothermictreatment of siliceous feed material, the apparatus including: acylindrical housing extending about a central axis and having a firstand second ends, and a central mixing zone adjacent the first end; amaterial intake for carrying said feed material into the mixing zone; afirst gas inlet at said first end and having guides to create an innervortex extending about said axis; a second gas inlet at said second endand having guides to create an outer vortex extending about said axis,the inner and outer vortices rotating in the same direction about saidaxis and moving axially in opposite directions, and the first and secondvortices creating a gas stream; a gas outlet positioned centrally insaid second end to receive spent gas from the housing; a control systemhaving temperature sensors in the gas stream and operable in response tochanges in temperatures in the gas stream to limit the temperature inthe feed material in the housing; whereby in operation gases are fedinto the first and second gas inlets carrying sufficient oxygen into thegas stream for exothermic combustion of the feed material and having aflow rate to create said inner and outer vortices, the outer vortexmeeting the inner vortex in the mixing zone to both mix with the innervortex and with the feed material leaving the material intake, so thatthe feed material will be subjected to exothermic combustion as the feedmaterial is entrained in the gas stream to pass through the mixing zonerepeatedly until the feed material is converted to a predominantlyamorphous silica ash having escape criteria needed to move in the gasstream to the gas outlet with the spent gas.
 21. A method as claimed inclaim 20 in which the ash has a silica ash content of at least 97% byweight.
 22. A method as claimed in claim 20 in which the ash has asilica ash content of at least 98% by weight.